It is well established that host immune defenses come into play at various stages of human disease. During viral infection, for example, antibodies stimulated in response to previous immunization may neutralize incoming viruses prior to attachment and penetration of susceptible target cells. In the event that cells become infected and display virus-associated antigens on their surfaces, cellular immune responses may also be activated. In this latter case, cytotoxic T cells can kill infected cells, thereby limiting progression of the infection. These humoral and cellular immune responses are commonly mounted against infection by a wide variety of viruses, including viruses having DNA or RNA genomes and outer coats composed of protein capsids or membrane envelopes.
The fact that animals can mount vigorous immune responses to most foreign antigens without similarly responding to components of their own tissues suggested to Burnet and Fenner (The Production of Antibodies, Macmillan Co., Melbourne (1949)) that the immune system must have evolved some mechanism for distinguishing self from non-self. A state of self-tolerance undoubtedly exists for central antigens to which the immune system is normally exposed. (See Siskind, G., Fundamental Immunology ed. W. E. Paul, Raven Press, New York, Ch. 20 (1984)). A “central antigen” is a self antigen that ordinarily is exposed to cells of the immune system, whereas a “peripheral antigen” is a self antigen that ordinarily is shielded from contact with cells of the immune system, for example by physical separation. Failure of the immune system to mount responses against certain components of the eye, brain and testes, for example, results from segregation of these tissues from the host immune system rather than from self-tolerance. Indeed, autoimmune responses can occur when the physical “barriers” that maintain these peripheral tissue antigens separate from immune surveillance are compromised. Remarkably, the vertebrate genome possesses all of the information needed to produce antibodies directed against a self antigen; and spontaneously generated antibodies to many self antigens can routinely be detected. However, these antibodies are low titer, low avidity and of the IgM class.
Several investigators believe that self-tolerance involves the immune system “learning” to distinguish self and non-self components, an event that occurs before maturing at around the time of birth. It has been speculated that exposure of the lymphoid system to self antigens during fetal development, for example, is a critical phase for developing tolerance to self antigens. According to other models, lymphocytes expressing cell surface receptors specific for the self antigen are eliminated, rendered incapable of activation, or are “tolerized” to the antigen.
The term “B cell tolerance” is often used to describe a state in which the immune system ineffectively responds to the presence of an antigen (e.g., a self antigen) or, more particularly, when the B cells of the immune system fail to mount a response to an antigen. Accordingly, an antigen that is normally exposed to B cells yet fails to induce a high titer antibody response or that is associated with a normal non-response by B cells (e.g., a self antigen) is referred to as a “tolerogen” because the immune system “tolerates” its presence. Clearly, self antigens are tolerogens but foreign antigens can also become tolerogens when B cells fail to sufficiently respond to the antigen. Some investigators believe, for example, that chronic viral infections occur (e.g., viral persistence in infants born to Hepatitis B virus (HBV) carrier mothers) because the immune system has become tolerized to viral antigens. (Takashima et al., 1992 Immunology, 75:398). Tolerogens are not necessarily entire molecules but can be portions of molecules (e.g., peptide fragments of proteins), in potentially immunodominant regions of a molecule. Although investigators have had success in inducing tolerance in animals by various techniques, our understanding of ways to generate antibodies to tolerogens is in its infancy.